Solid electrolyte capacitors with a niobium pentoxide or tantalum pentoxide dielectric have found wide use in the electronics industry. They are based on the high dielectric constants of niobium pentoxide and tantalum pentoxide, which forms the insulator layer in such capacitors. The anodes are produced by sintering finely divided porous agglomerates of tantalum or niobium primary particles to give porous anode bodies with correspondingly large surface area, the sintering being preceded by processing the agglomerates to give dimensionally stable pressed bodies which are then sintered under high vacuum at a temperature of 1000 to 1500° C. over 10 to 30 minutes. The pentoxide layer on the surface of the sintered bodies (anodes) is then obtained by electrolytic oxidation (i.e., by forming/anodizing/anodically oxidizing), the thickness of said layer being determined by the maximum voltage of the electrolytic oxidation (forming voltage). The cathode is obtained by impregnating the sponge-like structure with manganese nitrate, which is converted thermally to manganese dioxide, or with a liquid precursor of a polymer electrolyte and polymerization, or a polymer dispersion of a conductive polymer.
Niobium suboxide, NbOx where x=0.5 to 1.7, is in principle also suitable as an anode material owing to its high conductivity. Accordingly, there have recently been various proposals to use NbO as a replacement for niobium or tantalum metal as an anode material. The industrial use of NbO as an anode material has, however, not to date been possible because capacitors produced therefrom have too high a residual current, and a complete anode produced in the same way exhibits an extremely broad distribution of the specific residual current. Many capacitors from one production batch have a tendency to suffer voltage breakdowns, making them unusable.
The reason for the poor performance of capacitors with NbO anodes appears to lie in a defect structure obtained as a result of the production process: in contrast to the ductile metals Nb and Ta, NbO is a comparatively brittle oxide ceramic which, during the shaping of the anode, apparently suffers defects, for example, in the form of microcracks, which only heal incompletely even in the course of sintering.
It has now been found that such defects do not occur when the shaping of the anode body before the sintering is effected at ambient pressure.